The invention relates to determination of the spatial distribution of substantially mono-energetic neutral radiation, particularly X-rays, emanating from a point source and has potential application in the fields of X-ray diffraction, nuclear medicine and nuclear physics. For instance, it is suitable for use in detection of the spatial distribution of X-rays diffracted by a crystal or received through a pin-hole collimator.
Multiwire gas-filled proportional counters have been used for X-ray imaging. In such counters, each X-photon is converted into a burst of electrons due to the avalanche caused by the ionisation electrons in the vicinity of the counterwire. However, satisfactory yield makes it necessary to provide a gas volume of substantial thickness. If flat electrodes are used, there occurs substantial errors and uncertainties regarding the location of the ionising events caused by X-rays directed at an angle from the axis of the counter.
For overcoming that difficulty, it has been suggested to use cylindrical or part-spherical electrodes, as described for instance in U.S. Pat. No. 3,786,270 (Borkowski et al) and a paper by G. HARPAK et al in Nuclear Instruments and Methods 122 (1974) 307-312, North Holland Publishing Co. Then the electrons drift radially in the same direction as the incident X-rays and imaging may be satisfactory. On the other hand, construction of the detector is rendered more complex.
It is an object of the invention to provide an improved device for determining the spatial distribution of neutral radiation originating from a point source, such as a diffracting crystal or a pin-hole collimator, based on photon-electron conversion and avalanche multiplication.
It is another object to provide a device which is simple in construction and has satisfactory resolution, accuracy and efficiency.
A device according to the invention comprises a gas filled enclosure having a flat entrance window transparent for the radiation to be detected, typically X-rays. Flat field electrode means establish in a first portion of the enclosure an electrical field of such amplitude that conversion of said radiation occurs and avalanche electron multiplication results in delivery of a pulse of electrons per conversion whose pulse height is an increasing function of the travel path of the electron avalanche from the location of the conversion to the outlet field electrode of said first portion. Detector means located in a second portion of the enclosure are arranged to receive said pulse of electrons through said outlet electrode and to determine the coordinates of the pulse in said outlet electrode and the pulse height of said pulse.
While such a construction is simple, it makes it possible to compute the depth of the ionizing event in the first portion and to remove the uncertainty due to the lack of that indication in prior art systems having flat electrodes. Computation of the location and of the angular position of the original X-ray can be made using conventional analogue or digital electronics.
If the incident photons have a sufficient energy (typically 10 keV or more), sufficient amplification may be obtained in the conversion space for obtaining pulse heights which may be subjected to pulse height analysis and determination of the centroid and thereby provide acceptable resolution. For lower energy however, an electron drift or transfer space will be provided between the first and second portions. Electron transfer will occur through that space with a sufficient yield (typically 20 to 40%) and substantially linear amplification if the gas has been property selected. It has been found that a mixture of 95% Ar-5% C.sub.3 H.sub.8 or Xe-(C.sub.2 H.sub.5).sub.3 N generally gives satisfactory result.
The detector means may use any one of a number of well-known approaches providing a predetermined amplification factor; it may consist of a multiwire proportional chamber associated with delay lines, current dividing circuits, analogue centroid computation circuits, digital computers, etc. Determination of the depth Z of the ionising events may then be made based on a measurement of the pulse height and used for determining parallax. Another method, using parallel wires at different levels, may also be used.
Other aspects, advantages and features of the invention will appear from the following description of a particular embodiment, with reference to the accompanying drawings.